The invention relates to a phoropter for determining deficiency of sight of the human eye and the prescription of vision aids, respectively. In known devices for examination of the human eyes spherical lens mounts and frame elements for cylindrical lenses are rotated by means of electrical rotation means, refer to DE 33 31 799 A1, for example, to render them operative in a required combination in a vision window. Typically, synchronous motors or stepper motors are employed as electrical rotation means to operate the mounts and the frames, respectively, via belt-driven guide rollers or gear wheels. The known rotation devices are material and space-consuming and, hence, are responsible for covering a large part of a patient's face by the phoropter used in the course of an examination.
A further known piezoelectric motor (DE 38 33 342 A1) comprises a drive member and a driven member friction coupled to the former; the drive member excites two orthogonal components of motion in the range of the friction contact, one of the components effects the proper drive and the other one the contact. The drive member includes two exciting systems which are electrically and magnetically operated, respectively, and are separated from one another resulting in an unduly large space-consuming mass.
In a further known piezoelectric motor at least two piezoelectric crystal yokes are arranged in a rotatably seated drive drum secured to a frame which yokes abut against the interior face of the drum (DE 41 27 163 A1). These arrangements are very expensive and not applicable for phoropters.